The inhibition or elimination of rust formation has been a coatings goal for many years. Rust is formed by the oxidation of iron to form oxides of iron. Unlike some natural oxides such as aluminum oxide, which forms a fairly adherent oxide coating on the surface of aluminum, iron oxides are porous and the oxides do not adhere well to the underlying iron-based substrate. Thus iron-based substrates continue to form oxides in the presence of oxygen and water, eventually damaging the underlying structure.
Paints are one of the most cost effective coatings for iron structures. Various rust inhibitors are added to most paints; zinc chromate is particularly widely used. However, chromates must be used in fairly large quantities in paint to provide effective protection. Further, chromium is a heavy metal and thus poses a disposal problem. The costs of disposing of heavy metals such as chromium safely are increasing, and there are toxic dangers during disposal as well. However, in spite of environmental concerns, the elimination of chromates from paints has not been mandated until now because in truth there is no feasible substitute for chromates at the present time. Several other heavy metal compounds such as molybdates, phosphates and organic zinc compounds have also been tried, but they are much more expensive than zinc chromate, and they are much less effective in providing rust inhibition. Other heavy metals, such as lead, have already been banned due to their toxicity.
Other compounds have been suggested as corrosion inhibitors, particularly nitrites. Nitrites such as sodium nitrite are widely used to inhibit corrosion in water systems such as boilers, and they have been tried as rust inhibitors for paints as well, such as for flash rust inhibition in water based paints. In the short term, sodium nitrite is highly effective as a corrosion inhibitor, even at low concentration. For example, to inhibit the pitting of steel in water, about 65-160 ppm of chromate must be supplied, whereas only about 0.5-5 ppm of nitrite ion is equally effective. However, sodium nitrite is highly soluble in water, and thus is readily leached from the coatings over time, particularly in moist environments. Further, since sodium nitrite is highly soluble in water, it causes a paint coating to absorb water, which can result in blistering and lifting from the substrate by osmotic force as well.
Other metal nitrite salts, e.g., zinc nitrite, are also generally highly soluble in water, unlike metal chromates such as zinc or strontium chromate, which have low solubility in water. Thus metal nitrites cannot be substituted for zinc chromate in paint formulations.
More than forty years ago, organo amine nitrites were suggested as corrosion inhibitors for coatings. For example, U.S. Pat. No. 2,596,450 to Wachter discloses organo nitrites which are the reaction product of various amines and nitrous acid in a slightly basic mixture. While tertiary and quaternary amines are suggested, all of the Examples use secondary amines. These compounds, while they may be effective as corrosion inhibitors, can convert to nitrosamine compounds, which are known to be toxic, and typically are carcinogenic in even trace amounts. Thus such compounds cannot be used as paint coatings in today's environmentally sensitive era.
Tertiary amine nitrites were suggested for use as corrosion inhibitors in coatings in U.S. Pat. No. 2,432,840 to Wachter wherein an object of the invention was to provide corrosion inhibitors which were soluble in both organic materials and water, without restriction of their solubility. Wachter had no understanding that the solubility of the amine nitrites would have any effect on their use in paint formulations.
The early disclosures by Wachter clearly have not provided environmentally safe, cost-effective anti-corrosion additives for paint coatings, and thus the search continues in order to find a substitute for zinc chromate.